Numbers and Quantities
![Image](https://assets.serlo.org/legacy/56fe2459b0d79_5575d451f8bdf35b3a137cea4fd7332d69a514d1.png)
Basic arithmetic operations
Number sets, arithmetic expressions and general arithmetic laws
![Number sets, arithmetic expressions and general arithmetic laws](https://assets.serlo.org/legacy/56fb7df4e3754_9ab468f2312f55d8663a8e4624b4a17a86bf09d6.png)
What is a subtrahend, what does the distributive law mean, and what are rational numbers? - You can find explanations and exercises for these and other questions here.
Divisors and prime numbers
![Divisors and prime numbers](https://assets.serlo.org/legacy/56fbcaee11deb_30af6906032a9a5c7c1e3b209a63ab8059a58870.png)
What a prime number is, what divisors are all about, and how to perform a prime factorisation - you can find explanations and exercises on such questions here.
Fractions and decimal numbers
![Fractions and decimal numbers](https://assets.serlo.org/legacy/56fb93861f6f1_9f2891074724fca8a53ea19bdd294ce684c988fe.png)
Quantities and Units
![Image](https://assets.serlo.org/legacy/56fa7f8bb5e17_8c787caf639333fc3e1aa7da3b148b89b8047f2d.png)
Mass and length, area, volume, time and currencies - real-life applications usually deal with quantities and units and not just abstract numbers. You can find explanations and exercises here.
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